1. Field of the Invention
The present invention relates to a hybrid type rotary electric machine such as a stepper motor, which achieves a high torque within a compact size, offers low vibration, and is suitable for office machines.
2. Description of the Related Art
JP-A-3-212149 (hereinafter, referred to as JP '149) discloses an exemplary hybrid type stepper motor in accordance with the prior art. This stepper motor includes a two-phase stator having eight main poles and a hybrid type rotor. Each main pole of the stator has inductor teeth at the tip thereof. The hybrid type rotor includes a pair of rotor magnetic poles each having fine teeth on its outer peripheral surface, and a permanent magnet arranged between the rotor magnetic poles. The permanent magnet is magnetized in the axial direction.
FIG. 8 shows one stator main pole 1 and a portion of the rotor magnetic pole 2 of the stepper motor disclosed in JP '149. Six inductor teeth are arranged at a regular pitch at the tip of the stator main pole, and includes a pair of innermost inductor teeth 1c and 1d in the central portion of the main pole, a pair of intermediate inductor teeth 1b and 1e on the outside of the innermost inductor teeth 1c and 1d, and a pair of outermost inductor teeth 1a and 1f on the outside of the intermediate inductor teeth 1b and 1e. The fine teeth 2a of the rotor magnetic pole 2 are arranged at a regular pitch on the outer peripheral of the rotor magnetic pole 2.
In the stepper motor of JP '149, the tooth pitch of the fine teeth 2a of the rotor magnetic pole 2 and the tooth pitch of the inductor teeth 1a to 1f of the stator main pole 1 are set to 7.2 degrees and 6.9 degrees in mechanical angle, respectively, in order to reduce distortion of the stiffness characteristics.
When the center line C of the stator main pole 1 is aligned with the center line of the space between two fine teeth 2a of the rotor magnetic pole 2, as shown in FIG. 8, the center lines of the six inductor teeth 1a to 1f of the stator main pole 1 are displaced from the center lines of the opposed fine teeth 2a by displacement angles θ1 to θ6, respectively. Assuming that the tooth pitch of the fine teeth 2a of the rotor magnetic pole 2, i.e., 7.2 degrees in mechanical angle corresponds to 360 degrees in electrical angle, θ1 to θ6 are calculated as follows.θ3=θ4=(0.3°/2)(360°/7.2°)=7.5°θ2=θ5=(0.3°+0.3°/2)(360°/7.2°)=22.5°θ1=θ6=(0.3°+0.3°+0.3°/2)(360°/7.2°)=37.5°
In this case, the fundamental component P1 of permeances of the six inductor teeth, which generates magnetic flux linkage and a motor torque, is calculated by Expression 1.
                                                                        P                ⁢                                                                  ⁢                1                            =                            ⁢                                                cos                  ⁢                                                                          ⁢                  θ                  ⁢                                                                          ⁢                  3                                +                                  cos                  ⁢                                                                          ⁢                  θ2                                +                                  cos                  ⁢                                                                          ⁢                  θ1                                +                                  cos                  ⁢                                                                          ⁢                  θ4                                +                                  cos                  ⁢                                                                          ⁢                  θ5                                +                                  cos                  ⁢                                                                          ⁢                                      θ                    ⁢                    6                                                                                                                          =                            ⁢                              2                ⁢                                                      (                                                                  cos                        ⁢                                                                                                  ⁢                        7.5                        ⁢                        °                                            +                                              cos                        ⁢                                                                                                  ⁢                        22.5                        ⁢                        °                                            +                                              cos                        ⁢                                                                                                  ⁢                        37.5                        ⁢                        °                                                              )                                    /                  6                                                                                                        =                            ⁢              0.902                                                          (                  Expression          ⁢                                          ⁢          1                )            Thus, 90.2% of the permeances of the six inductor teeth forms a torque component.
The fourth harmonic component P4 of the permeance of the six inductor teeth which generates a cogging torque is calculated by Expression 2.
                                                                        P                ⁢                                                                  ⁢                4                            =                            ⁢                                                cos                  ⁢                                                                          ⁢                  4                  ⁢                  θ                  ⁢                                                                          ⁢                  3                                +                                  cos                  ⁢                                                                          ⁢                  4                  ⁢                                                                          ⁢                  θ2                                +                                  cos                  ⁢                                                                          ⁢                  4                  ⁢                                                                          ⁢                  θ1                                +                                  cos                  ⁢                                                                          ⁢                  4                  ⁢                                                                          ⁢                  θ4                                +                                  cos                  ⁢                                                                          ⁢                  4                  ⁢                  θ5                                +                                                                                                      ⁢                              cos                ⁢                                                                  ⁢                4                ⁢                θ6                                                                                        =                            ⁢                              2                ⁢                                  (                                                            cos                      ⁢                                                                                          ⁢                      30                      ⁢                      °                                        +                                          cos                      ⁢                                                                                          ⁢                      90                      ⁢                      °                                        +                                          cos                      ⁢                                                                                          ⁢                      150                                                        )                                ⁢                °                                                                                        =                            ⁢              0                                                          (                  Expression          ⁢                                          ⁢          2                )            
FIG. 9 shows distribution of vectors V1 to V6 of the permeances of the inductance teeth 1a to 1f in the fourth harmonic plane where a mechanical angle of 7.2 degrees corresponds to an electrical angle of 360 degrees. As shown in FIG. 9, the sum of the vectors is zero. This means that the cogging torque is cancelled out, in theory.
U.S. Pat. No. 6,674,187 (hereinafter, referred to as U.S. Pat. No. '187) discloses another exemplary hybrid type stepper motor in accordance with the prior art in which two rotor units with a non-magnetic plate interposed therebetween are fixed to a shaft. Each rotor unit has a pair of rotor magnetic poles and a permanent magnet arranged therebetween. The two permanent magnets of the rotor units are magnetized in the same direction as each other.
In the stepper motor of JP '149, the cogging torque is canceled only when the permeances of the six inductance teeth of the stator main pole are substantially equal to one another. Actually, the cogging torque of the stepper motor of JP '149 cannot be zero because of a difference between the permeances. More specifically, a distance of each inductor tooth from the center of the main pole is different among the innermost inductor teeth, the intermediate inductor teeth, and the outermost inductor teeth. In addition, the condition of leakage of magnetic fluxes generated by each outermost inductor tooth 1a or 1f is different from those of other inductor teeth 1b to 1e because of air existing on one side of the outermost inductor tooth. For those reasons, all the permeances of the inductor teeth 1a to 1f are not the same.
Moreover, the torque of the stepper motor of JP '149 cannot be increased without increasing the size of the stepper motor of JP '149 in the radial direction, because the stepper motor only includes a single rotor unit. One approach to increase the torque is to provide two rotor units along the rotation shaft, as disclosed in US '187.
However, even in the stepper motor of US '187, it is difficult to achieve a sufficiently high torque with a low vibration for the following reasons. First, the rotor portion of the stepper motor of US '187 includes a non-magnetic plate for magnetically insulating two rotor units from each other. No torque is generated at the non-magnetic plate, and the magnetic fluxes of the magnetic circuit which pass through those two rotor units are weakened by each other around the non-magnetic-plate. Therefore, a sufficiently high torque cannot be obtained. Second, the non-magnetic plate has to be thick enough to prevent the magnetic flux leakage. This may increase the cost of the rotary electric machine.
Furthermore, in a case where a plurality of rotor units are provided as in the stepper motor of US '187, the stator must be thick in the axial direction and the cogging torque caused by the magnetic fluxes of the permanent magnets also increases. This cogging torque may cause vibration during the motor rotation or degrade the positioning accuracy.